Increasingly, the drilling of oil and gas wells is no longer a matter of drilling vertically straight boreholes from the surface to a zone of hydrocarbon recovery using a traditional drilling platform. Such platforms are surmounted by a derrick which supports a string of jointed drill pipe with a bit at the lower end of the pipe. Technology and techniques have now been developed to change the angle of the bore's trajectory by up to and sometimes exceeding 90.degree. from the vertical. Directional drilling using coiled tubing rather than jointed pipe can offer numerous advantages compared to conventional drilling including new approaches to oil and gas traps having non-conventional geometries, economic zone enhancement as can occur for example if the borehole is deviated to actually follow an oil or gas bearing strata, improved economics particularly in an under-balanced environment (when formation pressure is sufficient to force hydrocarbons to the surface at potentially explosive rates) and reduced environmental degradation.
After deviating a borehole from the vertical, it's obviously no longer completely practical to sustain continuous drilling operations by rotating the drill string in order to also rotate the bit. Preferably only the bit but not the string is rotated by a downhole motor attached to the lower end of the drill pipe, the motor typically consisting of a rotor-stator to generate torque as drilling fluid passes between the rotor and stator, a bent housing (sometimes also referred to as a bent "sub") to deviate the hole by the required amount and which also surrounds a drive shaft that transmits the rotor/stator's torque to a bearing assembly, and a bit rotatably supported at the downhole end of the bearing assembly for cutting the borehole. The bent housing causes the centre line of the components downhole of its position, including the bit, to be at a different angle to the centre line of the components on its uphole end, including the drill string. As the drilling continues, the bit will therefore deviate from the vertical in a curved path in the direction of the bend of the bent housing.
Electronic means supported by a mule shoe in the bottom hole assembly and connected to the surface by a wire line passing through the interior of the drill string transmits information with respect to the amount of curvature in the borehole's trajectory so that it can be plotted. Once the required curvature has been attained so that the axis of the bit's rotation is pointed in the desired direction, the drilling is stopped and the motor must be withdrawn from the well. The bent housing is then either removed or straightened (if it's of the adjustable sort) and the motor is tripped back into the hole to resume drilling. Each time the motor requires service, or a change in the hole's direction is required, this process must be repeated. This results in substantial costs and down time largely due to the time required to make and break all of the joints as the drill string is tripped in and out of the hole. For this reason, jointed drill pipe is now being replaced whenever possible with coiled tubing.
In addition to controlling the bend angle in the coil tubing, it is also necessary to orient the bend point to control and adjust the borehole's bearing or azimuth. Examples of orienting tools for controlling azimuth are disclosed in U.S. Pat. Nos. 5,311,952, 5,316,094, 5,215,151 and 5,373,898 and in U.K. Application Serial No. 2,271,795A.
The orienting tool described in U.S. Pat. No. 5,311,952 includes an upper tubular housing that remains in fixed orientation relative to the coiled tubing. A mandrel is slidably and rotatably received within the upper housing, the mandrel including a piston at its upper end. The piston and mandrel are normally biased into a first position by a coil spring. A lower tubular housing extends from the lower end of the upper housing and is coupled to a mandrel by means of splines so that it rotates with the mandrel. The lower housing therefore rotates relative to the upper housing but cannot move longitudinally relative thereto. A lug and lobe indexing system is employed to turn housing 56 in predetermined increments in response to up and down movements of the mandrel. A nozzle is provided at the bottom of the mandrel. At normal flow rates of the drilling mud, the pressure drop across this nozzle is sufficient to force the mandrel down into its lower position against the force of the coil spring. If the mud flow is reduced sufficiently, spring 40, assisted by reactive torque from the bit, will raise the mandrel to its upper position. This up and down movement of the mandrel engages the indexing system to incrementally rotate the mandrel 45.degree. for each up and down cycle of the mandrel. This tool therefore cannot be activated independently of the drilling fluid, is rotationally unidirectional, and positional control of less than 45.degree. increments is possible only by means of regulating the weight-on-bit (WOB) requiring complicated methodology.
U.S. Pat. No. 5,316,094 describes an orienting tool including an outer cylindrical housing and an inner rotatable mandrel that connects at the lower end of the housing with another portion connectable to a motor for imparting an orienting torque to the bent housing. The annulus between the mandrel and the housing encloses a piston which is non-rotatable relative to the mandrel. The piston is connected to a piston follower having a pin which travels in a 360.degree. degree helical groove in the mandrel. Movement of this pin in the longitudinal direction is translated into rotation of the mandrel which includes a ratchet mechanism which allows rotation in one direction only in 10.degree. increments. This tool is controlled by hydraulic fluid delivered from the surface through a single supply line. Hydraulic fluid is apparently delivered with sufficient pressure to overcome the force of a spring and the drilling fluid acting on the lower side of the piston to drive the piston down which rotates the mandrel. This tool rotates in one direction only and lacks infinite positioning control.
U.K. Application No. 2,271,795 describes a combination bent sub and orienting tool which is actuatable from the surface such as by means of hydraulic supply lines which permit operation of the tool independent of the flow of drilling fluids and with weight-on-bit. However, changes in the bend angle are also accompanied by changes in orientation which can produce unwanted deviations in the borehole trajectory.
It is therefore desirable to produce an orienting tool which is controlled independently from the mud flow rate. It is also desirable to eliminate the need for weight to be applied to the drill bit but which nevertheless is adjustable with weight-on-bit. It is further desirable to provide an orienting tool which can be rotated in either direction as required.